Spray pyrolysis is a robust and cost effective way of producing micron and nanoscale powders. The degree of control over the morphology of the resulting particles, however, has been limited. In particular, hollow spheres with a porous and fine structured shell are typically produced, especially when the desired particles are a few microns in size or more. “Morphology of single-component particles produced by spray pyrolysis,” S. Jain, D. J. Skamser, T. T. Kodas, Aerosol Sci. Technol., 27, 575 (1997); “Ceramic powder synthesis by spray pyrolysis,” G. L. Messing, S. C. Zhang, G. V. Jayanthi, J. Am. Ceram. Soc., 76, 2707 (1993). Such a structure is undesirable for applications such as energy storage where the packing density of a material is a key performance parameter. While several methods have been developed to address the problem of hollow spheres, each has its own shortcomings and limitations.
For example, certain methods have utilized temperatures that exceed the melting point of at least one constituent or compound of the final, “fired” product, or evaporating the pyrolyzed material and then reconstructing a solid particle from vapor phase. “Ceramic powder synthesis by spray pyrolysis,” G. L. Messing, S. C. Zhang, G. V. Jayanthi, J. Am. Ceram. Soc., 76, 2707 (1993); “Flame spray pyrolysis: An enabling technology for nanoparticles design and fabrication,” W. Y. Teoh, R. Amal, L. Maedler, Nanoscale, 2 (8), 1324 (2010); “Flame aerosol synthesis of smart nanostructured materials,” R. Stroble, S. E. Pratsinis, J. Mater. Chem., 17 (45), 4743 (2007). Although addressing the hollowness, at least in part, these processes tend to result in non-porous or low porosity particles and tend to have undesirable phases and/or compounds due the high temperatures used during production.
Another method involves the use of drying control chemical additives (DCCA). “Effects of amide types DCCAs on the properties of Y2O3: Euphosphor powders with spherical shape and fine size,” H. Y. Koo, S. H. Lee, C. Y. Kang, J. Ceram. Soc. Jpn., 116 (1357), 955 (2008); “Effects of types of drying control chemical additives on the morphologies and electrochemical properties of Li4Ti5O12 anode powders prepared by spray pyrolysis,” S. H. Ju, C. Y. Kang, J. Alloys Compd., 506 (2), 913 (2010). The inclusion of such additives with the precursor was reported to affect the morphology of particles and prevent hollow structure for particles of up to one or two microns but have not been effective for larger particle sizes. In addition, these additives are generally expensive and present the risk of contaminating the final product.
Yet another method involves spray drying of milled product. “Synthesis of spherical LiMn2O4 microparticles by a combination of spray pyrolysis and drying method,” I. Taniguchi, N. Fukuda, M. Konarova, Powder Technol., 181 (3), 228 (2008). This multi-step path offers the advantage of relatively large secondary particle, but the particles consist of multiple fragments with no bonding connection or structural strength. Although this problem may be overcome by the addition of a binder to the sprayed solution, the binder itself may act as a contaminant in the product and may prevent intimate contact between fragments.
Still further, limited amounts of nanoparticles have been included in the precursor solution to serve as seeds that promote heterogeneous nucleation during the pyrolysis stage. “Photoluminescence optimization of luminescent nanocomposites fabricated by spray pyrolysis of a colloid-solution precursor,” W. Wang, W. Widiyastuti, T. Ogi, I. W. Lenggoro, K. Okuyama, Chem. Mater., 19 (7), 1723 (2007). The non-hollow particles produced by this method were only in the submicron size range. Furthermore, this method incorporates a large volume of non-active colloids into the final product, which is an undesirable feature for most applications.
In view of the foregoing, a need still exists for a method of producing non-hollow, porous particles of relatively large size (e.g., larger than one micron) without introducing undesirable impurities or phases.